Data Transmission Rate Adaptation in a Wireless Communication System

ABSTRACT

A method for controlling a data transmission rate of at least one transceiver in a wireless system, the transceiver including a transmitter and a receiver, the method including the steps of: determining a signal quality characteristic corresponding to a signal received at the receiver by measuring a difference between one or more reference constellation points and one or more received constellation points, the signal quality characteristic representing an estimation of signal degradation through the wireless communication channel; and modifying a data transmission rate of the transmitter as a function of the signal quality characteristic. The step of modifying the data transmission rate of the transmitter includes: determining lower and upper threshold levels representing reference minimum and maximum signal quality characteristics, respectively, corresponding to the data transmission rate; measuring a signal quality characteristic of the received signal; determining whether the measured signal quality characteristic is within the lower and upper threshold levels; maintaining the data transmission rate when the measured signal quality characteristic is between the lower and upper threshold levels; and increasing the data transmission rate when the measured signal quality characteristic is less than one or more lower threshold levels associated with one or more corresponding higher data rates.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.10/305,554 filed on Nov. 27, 2002, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to communication systems, andmore particularly relates to controlling a data transmission rate in awireless communication system.

BACKGROUND OF THE INVENTION

In a conventional wireless communication system including a pair oftransceivers communicating with one another over a wirelesscommunication channel, there are typically a number of different datatransmission rates available at which to transmit data. Generally, thehigher the data rate, the more susceptible the system is to errors.Under certain circumstances, it is necessary to adapt the system tohigher or lower data transmission rates, depending on environmentalconditions. For example, noise on the communication channel, transceiverimpairments, etc., may necessitate operation of the system at a lowerdata transmission rate.

The Institute of Electrical and Electronics Engineers (IEEE) 802.11standard addresses medium access control over a wireless local areanetwork (WLAN). The IEEE 802.11 standard is set forth in the documentIEEE Std. 802.11, entitled Supplement to IEEE Standard for InformationTechnology—Telecommunications and Information Exchange BetweenSystems—Local Metropolitan Area Networks—Specific Requirements—Part 11:Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)Specifications, 1999 Edition, which is incorporated herein by reference.Additional extensions relating to the 802.11 standard, including IEEEStd. 802.11a, entitled High Speed Physical Layer in the 5 GHz Band,February 2000, and IEEE Std. 802.11g, entitled Further Higher Data RateExtension in the 2.4 GHz Band, September 2000, are also incorporatedherein by reference. Rate adaptation in a wireless communication systemoperating in accordance with the 802.11 standard generally takes placein the transmitter at the MAC level. Known rate adaptation techniquestypically rely on information acquired through acknowledgment messagesreceived after each correctly transmitted data packet.

An acknowledgment message indicates a correctly received packet, whilean absence of an acknowledgment message is generally interpreted as anerror. A determination as to whether to change the data rate in thetransmitter can be made in response to the number of consecutiveacknowledgments that are received. After a certain number of correctlyreceived data packets, the transmitter typically attempts to switch to ahigher data transmission rate. Similarly, after a certain number ofconsecutive errors, the transmitter attempts to switch to a lower datatransmission rate. This conventional rate-switching methodology, whichis based on received acknowledgments, has the advantage of simplicity.However, it often adapts the data transmission rate of the transmitterto a value that is either too high or too low, thus undesirablyimpacting the throughput of the system. For example, switching to alower data rate when, in fact, a higher rate can be supported by thesystem results in a significant throughput degradation. The same is truewhen switching to a higher data rate than the system can support, thusresulting in a high packet error rate (PER), bit error rate (BER), orframe error rate (FER).

There is a need, therefore, for an improved rate-switching technique forcontrolling the data transmission rate in a wireless communicationsystem, which address the above-mentioned problems exhibited inconventional wireless communication systems.

SUMMARY OF THE INVENTION

The present invention provides techniques for advantageously adapting adata transmission rate of a wireless communication system to varyingconditions in the system. Such varying conditions may include, forinstance, impairments in a wireless communication channel associatedwith the system, impairments in a transceiver communicating over thewireless communication channel, etc. According to the invention, adecision regarding whether or not to change the data transmission rateof the wireless system is based, at least in part, on an estimation ofsignal degradation through the wireless communication channel.

In accordance with one aspect of the invention, a method is provided forcontrolling a data transmission rate of at least one transceiver in awireless system, the transceiver including a transmitter and a receiver.The method includes the steps of: determining a signal qualitycharacteristic corresponding to a signal received at the receiver bymeasuring a difference between one or more reference constellationpoints and one or more received constellation points, the signal qualitycharacteristic representing an estimation of signal degradation throughthe wireless communication channel; and modifying a data transmissionrate of the transmitter as a function of the signal qualitycharacteristic. The step of modifying the data transmission rate of thetransmitter includes: determining lower and upper threshold levelsrepresenting reference minimum and maximum signal qualitycharacteristics, respectively, corresponding to the data transmissionrate; measuring a signal quality characteristic of the received signal;determining whether the measured signal quality characteristic is withinthe lower and upper threshold levels; maintaining the data transmissionrate when the measured signal quality characteristic is between thelower and upper threshold levels; and increasing the data transmissionrate when the measured signal quality characteristic is less than one ormore lower threshold levels associated with one or more correspondinghigher data rates.

These and other features and advantages of the present invention willbecome apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an exemplary wirelesscommunication system in which the techniques of the present inventionmay be implemented.

FIG. 2A is a block diagram depicting an illustrative methodology fordetermining a signal degradation (SD) indicator, in accordance with oneaspect of the present invention.

FIG. 2B is a block diagram depicting an illustrative methodology fordetermining an SD indicator, in accordance with another aspect of theinvention.

FIG. 3 is a graphical representation illustrating simulation results ofreference SD curves for varying signal-to-noise ratios (SNR) and timedelay spread (TDS), in accordance with the present invention.

FIG. 4 is a graphical representation illustrating simulation results ofSD deviation for 50 and 100 nanoseconds (ns) TDS and varying SNRs, inaccordance with the present invention.

FIG. 5 is a graphical representation illustrating simulation results ofsystem performance at different data rates, in accordance with thepresent invention.

FIG. 6 is a graphical representation illustrating simulation results oflower and upper threshold SD levels, in accordance with the presentinvention.

FIG. 7 is a state diagram depicting an exemplary rate switchingmethodology, in accordance with one aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described herein in the context of an IEEE802.11 compliant orthogonal frequency division multiplexing (OFDM)wireless communication system. It should be appreciated, however, thatthe present invention is not limited to this or any particular wirelesscommunication system. Rather, the invention is more generally applicableto techniques for more optimally controlling a data transmission rate ina wireless system. Also, although particularly well-suited for use inconjunction with the IEEE 802.11 standard, the invention can be usedwith other standards, as well as in non-standard systems.

FIG. 1 depicts an exemplary wireless communication system 100 in whichthe methodologies of the present invention may be implemented. Theexemplary wireless communication system 100 includes a pair oftransceivers 102 and 104 communicating with one another via acommunication channel 106 established between the two transceivers 102,104. Channel 106 may be a wireless communication link, such as, but notlimited to, radio frequency (RF), infrared (IR), microwave, etc.,although alternative communication media may be employed. Transceiver102 preferably comprises a receiver 108 for receiving signals from thechannel 106, and a transmitter 110 for sending signals over channel 106.Similarly, transceiver 104 comprises a receiver 114 and a transmitter112. Receivers and transmitters suitable for use with the presentinvention are well known by those skilled in the art. Accordingly, adetailed discussion of such receivers and transmitters will not bepresented herein.

In accordance with the invention, a signal quality estimation of areceived baseband signal can be useful for adapting the datatransmission rate of transmissions over the wireless communicationchannel 106. Therefore, in accordance with one aspect of the invention,a signal degradation (SD) characteristic is preferably determined at areceiver side, exemplified by receiver 108, in a given transceiver,exemplified by transceiver 102. The SD characteristic, which representsan estimate of the signal quality through the communication channel 106,is made available to the corresponding transmitter 110 in thetransceiver 102, since the transmitter typically sets the datatransmission rate of transmissions over the channel 106. The receiver108 preferably derives the SD characteristic by processing an incomingmessage, which can be, for example, a data frame or a control frame(e.g., an acknowledgment message). Channel impairments which mayundesirably affect the ability of a signal to pass through the channelinclude, for example, co-channel interference, delayed signalinterference, narrowband interference (e.g., from intermodulationproducts), thermal noise, etc. Assuming quasi-static symmetric channeltransfer characteristics and transceiver impairments, acknowledgmentmessages undergo essentially the same signal degradation as the actualdata sent and thus will be substantially the same in quality.

In multi-carrier systems, as well as single-carrier systems, eachreceived frame generally includes a preamble and/or header portion. Thepreamble is used primarily for synchronization purposes, while theheader is primarily used for, among other things, specifying a lengthand rate of the payload data. Typically, the preamble and header aremodulated and encoded in a fixed manner, which may be simple and robustin comparison to the payload data in order to ease synchronization andreception of the transmitted data frame. For example, in an IEEE802.11a/g OFDM multi-carrier system, the preamble and a SIGNAL field inthe header may be modulated using binary phase shift keying (BPSK) andencoded using a binary convolutional code (BCC) rate one-half (½)encoder. Since the SIGNAL field is always modulated and encoded in thesame manner, this information can be advantageously used to derive an SDindicator that is substantially independent of the payload data. The SDindicator may be determined, for example, by measuring a Euclideandistance (i.e., a straight line distance) between known referenceconstellation points and received constellation points of the SIGNALfield, in accordance with the invention. The closer the receivedconstellation points are to the reference constellation points, thebetter the signal quality is, and vice versa. Other distance measurescan also be used.

In the illustrative embodiment of the invention, the reliability of theSD indicator depends on at least two factors, since the use of theSIGNAL field alone may limit the precision of the signal qualityestimation methodology. Therefore, in addition to use of the SIGNALfield, the signal quality estimation may also be based on, for example,an amount of variation and/or symmetry in the communication channelmedium, or additional and/or alternative characteristics. Adetermination of the variation in the communication channel is useful inthat a fast varying channel often causes the signal quality to changewithin a packet. Likewise, a determination of channel symmetry is usefulin that an asymmetric channel tends to cause the signal quality of thetransmitted packet to deviate from the received packet. A detaileddescription of the signal quality estimation methodology of theinvention is presented below in the case of an exemplary IEEE 802.11a/gmulti-carrier system. For ease of explanation, a symmetric and constantcommunication channel over a given packet is assumed.

As previously discussed, in conventional systems, adapting the data rateof the system primarily relies on information acquired throughacknowledgment messages received in response to a transmitted datapacket. The data transmission rate of the communication channel isusually changed depending on the number of good or badtransmitted/received packets. On the transmitter side, a receivedacknowledgment message is interpreted as a correctly received packet,while the absence of a received acknowledgment is interpreted as anerror. Typically, when a predefined number of packets are received witherrors, the transmission rate is switched down by one rate level. Thisprocess continues until a valid acknowledgment is received. Similarly,increasing the transmission rate typically occurs after receiving apredefined number (e.g., five) of acknowledgments. In this instance, thetransmitter generally attempts to send a packet at a higher transmissionrate. When an acknowledgment is received, the transmitter will change tothe higher rate, while absence of an acknowledgment will result inkeeping the lower rate.

Using conventional rate-switching approaches, the transmission rate isoften undesirably lowered too quickly, especially in high-density areas.This may be attributed to a higher probability of collisions occurringbetween different stations which often cause acknowledgment messages tobe missed. As previously explained, a missed acknowledgment message is,in this case, incorrectly interpreted by the transmitter as an error,thus undesirably initiating the rate-switching procedure. Accordingly,the present invention advantageously provides an improved rate-switchingmethodology which allows the system to more optimally switch thetransmission rate over the channel and is more reliable thanconventional rate-switching approaches. Moreover, the present inventionis not limited to rate-switching in single level increments, but mayselectively change the rate in larger (or smaller) increments asdesired.

In an exemplary IEEE 802.11a/g multi-carrier system, rate adaptationtakes place inside the transmitter at a medium access control (MAC)layer. As previously stated, in accordance with an illustrative aspectof the invention, a representative SD indicator is determined,preferably at the receiver side, and is provided to the correspondingtransmitter associated with a given transceiver. The SD indicator is ameasure of signal quality corresponding to a received signal (e.g., apacket) and preferably estimates a condition of the communicationchannel at a given time. The receiver can measure the signal quality,for example, by processing an incoming message, which may includepayload and/or acknowledgment data. When assuming quasi-static symmetricchannel transfer characteristics and transceiver impairments, theprocessed message will undergo substantially the same degradation as theactual data sent, and thus the two will be substantially equal in termsof signal quality. The transmitter then bases its rate-switchingdecision, at least in part, on the SD indicator.

FIG. 2A illustrates a block diagram of an exemplary circuit 200 forimplementing a methodology (e.g., Signal Processing Worksystem (SPW)implementation) for computing the SD indicator, in accordance with oneaspect of the invention. Circuit 200 may be implemented in the receiverof a given transceiver. Alternatively, circuit 200 may be implementedexternally to the receiver, such as being incorporated into thetransmitter or in a separate section of the transceiver, e.g., acontroller (not shown). The SD indicator determination methodologypreferably involves measuring a Euclidean distance (i.e., straight linedistance) between reference constellation points and receivedconstellation points corresponding to the modulated input signal,although alternative techniques are also contemplated by the invention.As previously stated, the closer the received signal constellationpoints are to the reference constellation points, the better the signalquality is, and vice versa. For rate-independent processing, and forease of explanation, only the SIGNAL field of a message is used in theSD indicator measurement. It is to be appreciated, however, thatadditional and/or alternative portions of the input signal may be usedfor computing the signal quality estimation, in accordance with theinvention. According to the above-cited 802.11a and 802.11g extensionsto the 802.11 standard, the SIGNAL field includes 24 bits that are rate½ coded and BPSK modulated, resulting in 48 samples located at phases of+1 or −1, as will be understood by those skilled in the art.

As apparent from the figure, input samples x associated with the SIGNALfield of a message are scaled by amplitude correction samples y(amplitude_cor) which represent an amplitude estimate of the channel andpower droop, among other characteristics, at block 202. The scalingprocess in block 202 is performed, at least in part, to align the inputsamples x with corresponding reference samples. The scaled samples x/yare then fed to separate blocks 204 and 206 where they are compared withthe reference samples at phases of +1 and −1, respectively. Thecomparisons performed at blocks 204 and 206 may include, for example,summing the scaled samples with respective signals (1.0+0j) and(−1.0+0j) to generate respective error samples, each of which mayinclude in-phase (I) and quadrature-phase (Q) components.

Magnitudes of the resulting error samples corresponding to phases +1 and−1 are subsequently computed at blocks 208 and 210, respectively. Themagnitudes are preferably determined by taking a square root of thesquared I and Q components, as known by those skilled in the art. Thetwo error magnitude signals are compared at block 212 to determine whichsignal path contains the smallest error magnitude. The output of block212, which represents the minimum error magnitude value of the twosignal paths, is then further processed. The minimum error magnitudevalue is then preferably stored, for example, in an array at block 214.Block 214 may include a serial-to-parallel converter, or alternativemeans, for storing and/or arranging the minimum error magnitude valuescorresponding to each of the samples in the packet. After all 48 bits ofthe SIGNAL field have been processed at block 214, the 48 magnitudevalues are then summed at block 216 and the resulting number may be usedto represent the signal degradation.

In FIG. 2B there is shown an alternative embodiment of the SD indicatorcomputation circuit depicted in FIG. 2A. The SD indicator circuit 250 isadvantageous in that it can be more easily implemented in an integratedcircuit (IC) device. As apparent from the figure, this implementationdoes not employ a divide operation (which is generally more difficult toimplement) and has only one signal path, rather than two. A firstsimplification that can be performed is mapping all incoming samples tothe positive half plane. This may be accomplished, for example, byconverting the incoming samples of the SIGNAL field into real (Re) andimaginary (Im) components at block 252 and taking an absolute value of areal component at block 254. This simplification is justified becausecomparing a sample in the negative half plane with the negative (−1)reference point is the same, at least in terms of magnitude, ascomparing a mirrored version of this sample with the positive (+1)reference point. The absolute value of the real component, which is alsoa real component, is then preferably combined with the imaginarycomponent at block 256 to generate a complex signal. Block 256 may beimplemented in accordance with a real/imaginary-to-complex converter,which may include, for example, a digital signal processor (DSP), aswill be understood by those skilled in the art.

Instead of comparing the incoming samples with +1 or −1 referencesamples, which require scaling in front (as in the circuit of FIG. 2A),the incoming samples in exemplary circuit 250 are compared with anamplitude reference for that specific subcarrier at block 262. Thecomparison at block 262 may comprise subtracting the amplitude referencefrom the incoming samples. The signal representing the amplitudereference may be formed by combining a real component (Re) amplitude_refand an imaginary component (Im) 260 of the amplitude reference at block258. Block 258 may include a real/imaginary-to-complex converter, whichmay be implemented in a manner consistent with block 256 previouslydescribed. Further reduction in processing complexity may be achieved,for example, when the magnitude is approximated by a first orderestimation or when the power is computed instead. The result of thecomparison at block 262 is an error signal comprising I and Qcomponents. A magnitude of the error signal is preferably obtained atblock 264. The magnitude of the error signal may be computed by taking asquare root of the squared I and Q components of the error signal, aswill be understood by those skilled in the art.

The error magnitude values corresponding to each of the samples in theSIGNAL field may be summed by an integrator 270 that is reset after eachSIGNAL field. The integrator 270 may include a summation block 266coupled to a delay block 268 which at least temporarily stores aprevious magnitude value. After all 48 magnitude values corresponding tothe 48 bits in the SIGNAL field have been summed by integrator 270, theresulting number may be used to represent the signal degradation.

An alternative methodology for determining the SD indicator may compriseprocessing the SIGNAL field samples as well as pilot samples. The pilotsamples, like the SIGNAL field samples, are preferably BPSK modulated,and can therefore be processed in the same manner as the SIGNAL fieldsamples. One advantage of this approach is that the signal degradationwould be determined using more than only the 48 samples of the SIGNALfield, and therefore may result in a more accurate estimate of thesignal quality of the corresponding packet. However, the pilot samplesare always spaced substantially the same in frequency, at least in anillustrative 802.11 implementation. It is to be appreciated that othercommunication systems may use pilot samples that are spaced differentlyin frequency throughout various symbol packets. Consequently, assumingonly pilot samples are employed, the resulting SD indicator would onlyestimate the signal degradation relating to those specific frequenciesand may therefore be undesirably affected by frequency selective fading.Computing the SD indicator using all frequencies would not be as proneto frequency selective fading. For at least this reason, a determinationof the SD indicator based on pilot samples alone may not be preferred.

By way of example only, simulation results for the illustrative SDindicator circuit 200 depicted in FIG. 2A will now be described.Although the alternative circuit 250 shown in FIG. 2B may providesomewhat different simulation values, the conclusions drawn herein maybe similarly applicable to both illustrative embodiments. For obtainingthe exemplary simulation results described herein, the incoming signalcomprises a SIGNAL field and data samples, with pilot samples alreadyremoved.

FIG. 3 is a graphical representation 300 illustrating exemplarysimulation results of reference SD curves for varying signal-to-noiseratios (SNR) and time delay spread (TDS) values in order to determinecorresponding reference/mean SD value for specific SNRs and TDSs, inaccordance with the invention. The exemplary simulation is carried outover 200 packets for several different SNR values (e.g., 6, 8, 10, 12,14, 16, 18, 20, 22, 24, 26, 28 and 30 decibel (dB)) and TDS values(e.g., 0 ns, 50 ns and 100 ns). It is to be appreciated that the numberof packets is arbitrary and may be chosen so as to provide an acceptablebalance between sample size and simulation speed.

The SIGNAL field of each packet may be processed according to theexemplary SD indicator determination methodology shown in FIG. 2A. Thiswill result in 200 different SD values for each different SNR value. Thereference mean SD value for a given SNR may be computed by averagingthese 200 SD values. FIG. 3 shows three reference SD curves 302, 304 and306 corresponding to three different TDS values, namely, 0 ns, 50 ns and100 ns, respectively. Each of the reference SD curves is graphed withrespect to the above-noted range of SNR values. As apparent from thefigure, there is about a 4 dB difference in SNR between curve 302 (0 nsTDS) and curve 306 (100 ns TDS) for an SD value of about 5. This impliesthat a system not suffering from TDS can handle about 4 dB more SNR thana system suffering from 100 ns TDS, both resulting in substantially thesame SD. The difference in SNR between curves 302 and 306 increasesslightly for lower SD values compared to higher SD values. Ideally, theSD of the SIGNAL field would perfectly match the SD of the total packet.However, this may not be the case in practice since the SIGNAL fieldrepresents just a portion of the total packet.

By way of example only, FIG. 4 is a graphical representation 400illustrating simulation results of SD deviation for a TDS of 100 ns andvarying SNRs, in accordance with the invention. From the simulationresults, it can be seen how accurately the SIGNAL field represents thetotal packet. The exemplary simulation is performed over 100 60-byteBPSK-modulated packets, which gives 21 payload symbols. When eachpayload symbol is processed in the same manner as the SIGNAL fieldsymbol, this results in an SD value for the total packet after averagingover all symbols. The total length of a packet in samples is determinedas:

(21 payload symbols+1 SIGNAL field symbol)×48=1056 samples

Since a static channel per packet is assumed, increasing the number ofsamples does not provide any significant additional information.

From the simulations, it can be shown that the distribution of theSIGNAL field SD compared to the total packet SD may be approximated by anormal distribution function. As will be understood by those skilled inthe art, a property of the probability density function (PDF) of anormal distribution is that about 95 percent of its samples lie within aμ±2σ boundary, where μ may be defined herein as the mean and σ may bedefined as the standard deviation of the distribution. The mean μ may benormalized to zero for every SNR and TDS value, but the standarddeviation σ is different. For a specific SNR and TDS value, there existsan SD reference value, and for that same SNR and TDS value, there is astandard deviation σ between the SIGNAL field SD and the total packetSD. Since the mean μ is zero, the standard deviation may be directlymapped to the SD reference values.

With continued reference to FIG. 4, the SD reference values ±2σ areshown for exemplary simulations with a TDS of 100 ns and SNR valuesranging from 6 dB to 30 dB. Reference SD curves 402 and 404 for a TDS of50 ns and 100 ns, respectively, are also shown. These curves 402, 404are the same as reference curves 304 and 306, respectively, shown inFIG. 3. The ±2σ boundaries 406, 408, 410, 412, 414 and 416 for aspecific reference SD value can be correlated to the SNR axis. Theresolution of the SD may be defined as a difference in SNR value betweenthe boundaries ±2σ. As apparent from the figure, the different SDdistribution regions may overlap, which implies that the resolution ofthe SD in this case is more than 4 dB SNR. Furthermore, the resolutionfor higher SD values is worse than the resolution for lower SD values.For instance, the SD resolution at an SD value of 6.81 is about 4 dBSNR, while the SD resolution at an SD value of about 34 is over 6 dBSNR.

Further information, upon which a data rate-switching decision may bebased, can be obtained from the simulation results by correlating the SDto system performance (e.g., packet error rate (PER)) curves. FIG. 5depicts exemplary simulated system performance curves a, b, c, d, e, f,g and h at data rates of 6, 9, 12, 18, 24, 36, 48 and 54 megabits persecond (Mbps), respectively, in accordance with the invention. Theexemplary simulation results were obtained for 1000 byte packets in afading channel with a TDS of 50 ns. As apparent from the figure, for aminimum PER of 5 percent (%) at a data rate of 54 Mbps, the SNR shouldbe greater than about 26.5 dB. As shown in FIG. 3, an SNR of 26.5 dB fora TDS of 50 ns corresponds to a reference SD value of about 3.4. Thissuggests that, on average, an SD value of 3.4 should result in a 5% PERfor a data rate of 54 Mbps.

Assume that an SD value of 3.4 results in a 5% PER at a rate of 54 Mbps,as noted above. Based on the exemplary simulation results, this impliesthat a measured SD value of 3.4 or lower is sufficient to achieve atleast a 5% PER for that specific data rate. However, as previouslyshown, the measured SD value may differ from the total packet SD, andtherefore employing a predetermined safety margin is recommended.

In conjunction with FIG. 4, exemplary simulation results were used tomeasure a deviation between the SD of the SIGNAL field and the SD of thetotal packet, from which a resolution bandwidth can be determined. Theseexemplary simulations illustrate that the resolution is worse for higherTDS values. The resolution results for the SD at a TDS of 100 ns canthus be seen as a worst case scenario for the resolution of the lowerTDS SD reference curves. Adding half of the resolution bandwidth to theSNR that yields a 5% PER for a specific data rate results in an SD valuewhich may be said gives at maximum a 5% PER with a certainty of 97.5%.Subtracting half of the resolution bandwidth results in an SD valuewhich may be said gives at minimum a 5% PER with a certainty of 97.5%.Accordingly, the two derived SD values may be seen as upper and lowerthreshold levels, respectively, for this specific data rate. Columns 4and 5 of Table 1 below provide exemplary lower and upper thresholdvalues, respectively, for corresponding data rates. These results arepresented graphically in FIG. 6.

TABLE 1 SNR @ Data Rate 5% Resolution Lower threshold Upper Threshold(Mbps) PER (dB) (dB) SD Value SD Value 54 26.5 4.0 2.6 4.2 48 24.5 4.53.3 5.3 36 20.75 5.0 4.8 8.4 24 15.5 5.5 8.5 16.0 12 10.25 6.0 15.2 30.06 7.0 8.0 19.6 —

FIG. 7 is a state diagram of an illustrative rate-switching methodology700, formed in accordance with one aspect of the present invention. Theillustrative rate-switching methodology 700 employs an SD indicatorwhich may be determined as previously described herein. As shown in thefigure, the illustrative rate-switching methodology preferably includesthree states, namely, a “Current Rate” state 702, a “Higher Rate” state704, and a “Lower Rate” state 706. Additional or alternative states arecontemplated by the present invention. It is assumed that the methodbegins in state 702. In state 702, a current data rate remains the sameand a lower threshold level L_Th and upper threshold level U_Th are setto the respective threshold levels of the corresponding current datarate.

When the measured SD value, as may be provided by the SD indicator, isgreater than or equal to the current lower threshold level and less thanor equal to the current upper threshold level, the exemplary method 700remains in state 702, thereby leaving the data transmission rateunchanged. When the measured SD value is below a lower threshold levelcorresponding to a higher data rate, a rate up-switching is performed.Thus, in accordance with an illustrative aspect of the invention,switching up a rate is performed by comparing the current SD value withthe lower threshold values of one or more higher data rates. When thecurrent SD value is below one of the lower threshold values associatedwith the higher data rates, the current data rate switches to thehighest data rate meeting this criteria. In this manner, it is possibleto advantageously increase the data rate by more than one step. In thisexample, the method 700 enters state 704. In state 704, the current datarate is preferably set equal to the higher data rate, and the upperthreshold level is set to the respective threshold level of thecorresponding higher data rate. Since the higher data rate essentiallybecomes the current data rate, the method may re-enter state 702 aftercompleting the above-noted modifications.

Rate down-switching may still rely on the occurrence of errors, but alsopreferably bases such rate-switching decision on past measured SDinformation. For example, the method may enter state 706 via a primarydecision path 708 when a predetermined number of consecutive errors(e.g., two) are detected and when a predetermined number of pastmeasured SD values are above the upper threshold level corresponding tothe current data rate. In state 706, the current data rate may be set toa lower data rate and the upper threshold value may be set to therespective threshold level corresponding to the lower data rate. As inthe rate up-switching case, the lower data rate essentially becomes thecurrent data rate, and thus the method 700 may re-enter state 702. Inaccordance with one aspect of the invention, rate down-switching, in amanner similar to the rate up-switching methodology previouslydescribed, may involve decreasing the data rate by more than one step.

The illustrative method 700 assumes that a rapid SD change does notoccur. Therefore, when the predetermined number of errors is detected,which would otherwise necessitate switching to a lower data transmissionrate, but past measured SD values are below the upper threshold levelfor the current data rate, the method may choose to keep the currentdata rate unchanged. In such a situation, the detected errors may beattributed to collisions rather than to degraded channel conditions.

To avoid a potential deadlock situation, wherein a relatively largenumber of consecutive errors are detected (thus reducing systemthroughput) but past measured SD values are still below the upperthreshold level corresponding to the current data rate, the method mayinclude a secondary decision path 710 from state 702 to state 706. Thissecondary path 710 may base the rate down-switching decision only on thenumber of consecutive errors detected, rather than on past measured SDvalues as well. For example, when a preset number of consecutive errorsoccur (e.g., five), the method may automatically enter state 706, thuslowering the data rate without checking past SD values. The number ofconsecutive errors detected for the secondary decision path 710 ispreferably set to be larger than the number of errors detected for theprimary decision path 708.

Simulations where the measured SD value is averaged over more than onepacket can also be performed. In this instance, the lower and upperthreshold levels may be closer together, which may make it easier todistinguish between the different data rates, at the expense of slowerrate switching. However, when averaging over more than one packet, it isimportant to consider the length of time between successive packets.When the time between packets is large, channel conditions can changedrastically, resulting in an SD value that does not accurately estimatethe actual channel conditions.

From the above exemplary simulations, it follows that a decision can bemade for rate-switching on the basis of the measured SD value. Inaccordance with another aspect of the invention, a less stringentdefinition of the SD resolution may result in relaxation of the upperand lower threshold levels. However, this may yield a less reliable rateestimation for the specific criterion of 5% PER. An alternative approachto relaxing the threshold levels may be to adopt a less stringent PERcriterion, for example, a 10% PER rather than a 5% PER.

In accordance with another aspect of the invention, a circuit forcontrolling the data transmission rate of the transceiver may include acontroller (not shown) that is configurable for performing at least aportion of the methodologies of the invention described herein. The termcontroller, as used herein, is intended to include any processingdevice, such as, for example, one that includes a central processingunit (CPU) and/or other processing circuitry (e.g., microprocessor).Additionally, it is to be understood that the term “controller” mayrefer to more than one controller device, and that various elementsassociated with a controller device may be shared by other controllerdevices.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade therein by one skilled in the art without departing from the scopeof the appended claims. For example, the invention can be used withstandards other than IEEE 802.11 (e.g., IEEE 802.15), as well as innon-standard applications.

1. In a wireless system comprising at least one transceiver configurablefor communication over a wireless communication channel, the transceivercomprising a transmitter and a receiver, a method for controlling a datatransmission rate of the at least one transceiver, the method comprisingthe steps of: determining a signal quality characteristic correspondingto a signal received at the receiver by measuring a difference betweenone or more reference constellation points and one or more receivedconstellation points, the signal quality characteristic representing anestimation of signal degradation through the wireless communicationchannel; and modifying a data transmission rate of the transmitter as afunction of the signal quality characteristic; wherein the step ofmodifying the data transmission rate of the transmitter comprises:determining lower and upper threshold levels representing referenceminimum and maximum signal quality characteristics, respectively,corresponding to the data transmission rate; measuring a signal qualitycharacteristic of the received signal; determining whether the measuredsignal quality characteristic is within the lower and upper thresholdlevels; maintaining the data transmission rate when the measured signalquality characteristic is between the lower and upper threshold levels;and increasing the data transmission rate when the measured signalquality characteristic is less than one or more lower threshold levelsassociated with one or more corresponding higher data rates.
 2. Themethod of claim 1, wherein the step of increasing the data transmissionrate further comprises the step of associating new lower and upperthreshold levels representing reference minimum and maximum signalquality characteristics, respectively, corresponding to the increaseddata transmission rate.
 3. The method of claim 1, wherein the referenceminimum and maximum signal quality characteristics are based on apredetermined minimum desired error rate and a maximum desired errorrate, respectively, of the at least one transceiver.
 4. The method ofclaim 1, wherein the step of measuring the difference between one ormore reference constellation points and one or more receivedconstellation points comprises: generating error signals for each of atleast a portion of data samples in the received signal, the errorsignals being a function of the differences between the receivedconstellation points and the corresponding reference constellationpoints; and determining a magnitude of the error signals, the magnitudeof the error signals representing the measured difference.
 5. The methodof claim 1, wherein the step of determining a signal qualitycharacteristic comprises measuring a difference between one or morereference constellation points and one or more received constellationpoints for at least one of data samples and control samplescorresponding to the received signal.
 6. The method of claim 1, whereinthe measured difference comprises a Euclidean distance.
 7. The method ofclaim 1, wherein the step of modifying the data transmission rate of thetransmitter further comprises decreasing the data transmission rate whenthe measured signal quality characteristic is greater than one or moreupper threshold levels associated with one or more corresponding lowerdata rates.
 8. The method of claim 1, wherein the step of determiningthe signal quality characteristic comprises: measuring a differencebetween one or more reference constellation points and one or morereceived constellation points for at least one of data samples andcontrol samples corresponding to the received signal by aligning the oneor more received constellation points with the one or more correspondingreference constellation points, the aligning step including the step ofscaling the one or more received constellation points by a prescribedscaling factor; and averaging at least a portion of the measureddifferences over a plurality of one of data samples and control samplesof the received signal, the signal quality characteristic being afunction of the resulting averaged difference, wherein the step ofaveraging at least a portion of the measured differences comprises thestep of adding a difference value corresponding to a present data sampleof the received signal and a difference value corresponding to aprevious data sample of the received signal.
 9. A circuit forselectively adapting a data transmission rate of a wirelesscommunication system, the wireless communication system comprising atransceiver configurable for communication over a wireless communicationchannel, the transceiver comprising a receiver and a transmitter, thecircuit comprising: at least one controller, the at least one controllerbeing operative to: (i) determine a signal quality characteristiccorresponding to a signal received at the receiver by measuring adifference between one or more reference constellation points and one ormore received constellation points, the signal quality characteristicrepresenting an estimation of signal degradation through the wirelesscommunication channel; and (ii) modify a data transmission rate of thetransmitter based, at least in part, on the signal qualitycharacteristic; wherein in determining the signal qualitycharacteristic, the at least one controller is operative to measure adifference between one or more reference constellation points and one ormore received constellation points for at least one of data samples andcontrol samples corresponding to the received signal by aligning the oneor more received constellation points with the one or more correspondingreference constellation points, the aligning step including scaling theone or more received constellation points by a prescribed scalingfactor; and wherein the at least one controller is further operative to:(iii) determine lower and upper threshold levels representing referenceminimum and maximum signal quality characteristics, respectively,corresponding to the data transmission rate; (iv) measure a signalquality characteristic of the received signal; (v) determine whether themeasured signal quality characteristic is within the lower and upperthreshold levels; (vi) maintain the data transmission rate when themeasured signal quality characteristic is between the lower and upperthreshold levels; and (vii) increase the data transmission rate when themeasured signal quality characteristic is less than one or more lowerthreshold levels associated with one or more corresponding higher datarates.
 10. An integrated circuit comprising at least one circuit forselectively adapting a data transmission rate of a wirelesscommunication system, the wireless communication system comprising atransceiver configurable for communication over a wireless communicationchannel, the transceiver comprising a receiver and a transmitter, the atleast one circuit comprising: at least one controller, the at least onecontroller being operative to: (i) determine a signal qualitycharacteristic corresponding to a signal received at the receiver bymeasuring a difference between one or more reference constellationpoints and one or more received constellation points, the signal qualitycharacteristic representing an estimation of signal degradation throughthe wireless communication channel; and (ii) modify a data transmissionrate of the transmitter based, at least in part, on the signal qualitycharacteristic; wherein the at least one controller is further operativeto: (iii) determine lower and upper threshold levels representingreference minimum and maximum signal quality characteristics,respectively, corresponding to the data transmission rate; (iv) measurea signal quality characteristic of the received signal; (v) determinewhether the measured signal quality characteristic is within the lowerand upper threshold levels; (vi) maintain the data transmission ratewhen the measured signal quality characteristic is between the lower andupper threshold levels; and (vii) increase the data transmission ratewhen the measured signal quality characteristic is less than one or morelower threshold levels associated with one or more corresponding higherdata rates.
 11. The integrated circuit of claim 10, wherein the at leastone controller is further operative to average at least a portion of themeasured differences over a plurality of one of data samples and controlsamples of the received signal, the signal quality characteristic beinga function of the resulting averaged difference.
 12. The integratedcircuit of claim 11, wherein the at least one controller is furtheroperative to: generate error signals for each of at least a portion ofthe data samples in the received signal, the error signals being afunction of the differences between the received constellation pointsand the corresponding reference constellation points; and determine amagnitude of the error signals, the magnitude of the error signalsrepresenting the measured difference.